Reactor fouling during polymerization reaction is due to the build-up of adherent polymer on internal surfaces, such as on the heat-transfer surfaces, of the polymerization reaction vessel. For example, in solution polymerization, these deposits may consist of polymer gels that are insoluble in the solvent used. The gel deposits impede the efficient transfers of heat, as well as result in the contamination of the reaction mixture and the products produced therefrom.
In emulsion polymerization, the build-up may be a coagulum formed because of excessive shear produced by the agitator or the build-up may be caused by the impingement of sticky semipolymerized droplets against the reactor wall. Such build-up decreases productivity and interferes with the quality of the end product. Thus, although the mechanism may differ depending upon the type of polymerization process being utilized, polymer build-up is a severe problem in the art of polymer production.
Over the years, a number of mechanisms and methods have been developed in an attempt to overcome the problems produced by polymer build-up. These have been produced with the aim of maximizing the rate of product output (i.e. the quantity of polymer per unit time) without sacrificing the most relevant polymer properties and concurrently minimizing costs.
For example different materials (i.e. polished steel, aluminum, glass, nylon, polyethylene etc.) have been utilized to line the reactor surfaces in order to avoid polymer adhesion. However, these materials have not been generally effective because in the absence cleaning of the reactor surfaces, polymer build-up still occurs. Furthermore, since polymerization of the vinyl chloride monomer takes place in an acidic environment due to the presence of vinyl chloride in the system which releases minute amounts of hydrochloric acid (HCl), the HCl so produced will attack the reaction vessel linings.
Consequently, cleaning of polymerization reactors has been accomplished for many years by manual scraping and chipping. However, this type of cleaning is not only costly with respect to labor and reactor/equipment downtime, it tends to damage the reaction vessel surfaces causing scratches and imperfections that tend to foul more quickly, resulting in more frequent cleaning schedules.
In addition, specially designed tools and mechanical scraping devices have been developed to minimize damage to the reactor surfaces. In some reaction vessels, automated apparatuses such as hydraulic cleaning devices have been installed. These mechanical devices can be utilized in conjunction with various chemical solvents which may be applied, i.e. such as sprayed onto the reaction vessel surfaces, to dissolve the polymer deposit. However, if the polymerization process is sensitive to such solvents, the reaction vessel must be washed and dried thoroughly after cleaning, resulting in additional reactor down-time.
In other reactors, spray rinse valves have been installed to allow the washing of the reaction vessel walls while the reactor is emptying. Additives may be applied to the reaction vessel walls in order to condition the walls for subsequent polymerization.
Along these lines, a number of coating and/or conditioning compositions have been produced for the purpose of inhibiting polymer build-up. These materials are normally sprayed or wiped onto the reactor surfaces before charging and are added to the reactor medium shortly after polymerization has begun in order to prevent, or reduce, fouling of the reactor wall.
For example, U.S. Pat. No. 4,080,173 teaches using self-condensation products of resorcinol as a reactor coating to prevent polymer build-up. U.S. Pat. No. 4,035,563 discloses using in the aqueous reaction medium, salts of nitrous acids such as sodium nitrite together with polyvalent metal salts as a system to reduce build-up on the walls of the reactor in a vinyl chloride polymerization process. U.S. Pat. No. 4,024,301 teaches using a coating composition consisting of an alkali metal hydroxide solution and a polyaromatic amine. U.S. Pat. No. 4,068,052 describes the use of dithiooxamide/aluminum mixtures to prevent build-up.
However, many of these materials are mildly toxic and often cause polymer odor and extrusion which is unacceptable for many applications. In addition, these methods and compositions are generally unacceptable because the amount of polymer build-up is insufficient and/or the properties of the polymerized product produced thereby are substantially altered.
Moreover, according to U.S. Pat. No. 4,659,791, the reactor coatings currently used in the polyvinyl chloride industry are primarily phenol materials such as self-condensed phenols or phenols condensed with other materials such as aldehydes or amines. Other aromatic compounds used as reactor coatings are thiodiphenols and their derivatives as well as poly aromatic amines (molecular weight range 200-1000). While these coatings have exhibited enhanced results in comparison to situations where no coatings have been applied, a number of major disadvantages remain including color deformation, odor, costs, durability, miscibility in reaction ingredients, thermal instability, inclusion of polymerization inhibitors, environmental hazard, etc.
The present invention is directed to the development of a more durable, more efficient and environmentally safer product that can be applied to reactor surfaces in order to substantially reduce polymer build-up. The polyaniline coating composition of the present invention exhibits excellent static discharge or electrostatic dissipation characteristics. In addition, as a result of its high degree of crystallinity, the polyaniline coating composition of the present invention exhibits excellent thermal stability when subjected to continuous operation at elevated temperatures (i.e. exhibits stability up to 700.degree. F.). The composition is insoluble in methanol and other general polymerization ingredients, is easily processed and inexpensive to produce and may be easily applied to reactor walls. Other objects of the invention may become apparent to those skilled in the art as the description proceeds.